1. Field of the Invention
The present invention relates to cameras, and more particularly to a camera having a backlight determination function and a wide-angle field distance-measuring camera having a passive auto-distance-measuring device that is used in cameras and video cameras and the like.
2. Related Art Statement
As cameras having a backlight determination function, a camera which measures the brightness of a primary subject using a photometric sensor to detect backlight has been known.
As the cameras having a backlight determination function, an area AF camera is known. In the area AF camera, the brightness of a primary subject, selected by an AF sensor, is measured using the AF sensor and the obtained brightness is compared to the average brightness obtained by an AE sensor, thus determining backlight.
In this type of camera, different sensors from the AF sensor and the AE sensor are used in order to accomplish the original objective of the AF sensor and that of the AE sensor. A dynamic range required for photometry is fairly large. Accordingly, in many cases, the respective sensors are generally designed so as to have sensitivity fairly close to the maximum dynamic range necessary for photographing.
In the above-mentioned camera based on the conventional techniques, the two different sensors necessarily have different photometrically possible ranges.
Particularly, many AF sensors generally use a method for integrating a photoelectric current of the sensor and obtaining the brightness of a subject on the basis of the time that elapsed before the integrated value reaches a threshold level. Accordingly, if the brightness is high, the photoelectric current is large, the time that elapsed before the integrated value reaches the threshold level is short, and a photometrical range for the high brightness is small.
In this instance, under photographing conditions that are fairly close to the maximum possible photometrical range or exceeding this range, backlight cannot be detected with accuracy. Disadvantageously, incorrect backlight detection is caused.
For example, for a subject area where the brightness of a subject exceeds the limit value of brightness which the AF sensor can measure but the subject brightness lies within a possible photometrical range of the AE sensor, the AF sensor exceeds the limit value of photometry. Consequently, the AF sensor outputs data which is different from the actual brightness of the subject, resulting in incorrect backlight detection. Although there is no backlight in fact, the camera may determine backlight by mistake.
On the other hand, AF cameras with a passive auto-distance-measuring device have been well-known. When this type of camera photographs a backlight scene or a scene in a nightscape as a background, integral action for measuring distance is performed using a high-brightness area of the background or a light source as a reference. Accordingly, a proper image signal of a person serving as a primary subject cannot be obtained. Disadvantageously, the person is out of focus but the background is in focus.
In order to overcome the above disadvantages, according to a distance-measuring method disclosed in Japanese Unexamined Patent Application Publication No. 5-264887, backlight is detected using photometric data and data of a distance-measuring sensor and, when a backlight mode is detected, a monitor area to monitor subject-brightness information, which is used for integral action, is set at the center of a capturing area. Alternatively, integration is performed using an area having the lowest integral speed of a plurality of preset monitor areas as a reference. Thus, poor auto-distance-measuring for the primary subject in the backlight mode is improved.
According to a distance-measuring method disclosed in Japanese Unexamined Patent Application Publication No. 7-199039, a nightscape mode for nightscape photographing is provided. When the nightscape mode is set, whether auxiliary light is needed is determined. When it is determined that brightness is low, auxiliary light is emitted to improve poor auto-distance-measuring in the nightscape photographing.
For the distance-measuring method disclosed in Japanese Unexamined Patent Application Publication No. 5-264887, however, when the high-brightness area of the background includes spotted light reflected by an object with high reflectivity, or when a primary subject such as a person exists on a position other than the center of the capturing area, backlight cannot be detected or image signals of a primary subject is saturated, so that the subject image which is out of focus may be captured.
For the distance-measuring method disclosed in Japanese Unexamined Patent Application Publication No. 7-199039, when a light source is bright in a nightscape as a background, auxiliary light may not be emitted. If the auxiliary light is emitted, in some cases, the amount of light may be insufficient and the effect of the auxiliary light may not be derived sufficiently.
In consideration of the above disadvantages, Japanese Unexamined Patent Application Publication No. 2001-141987 discloses that a first integral is performed on condition that a monitor area is set wider than conventional monitor areas, when the contrast of a low brightness area in output data of line sensors obtained by a first integral is low, the low brightness area is set to a monitor area, and a second integral is performed to this area. When a backlight scene is photographed or a scene in a nightscape as a background is photographed, image signals of a person serving as a primary subject can be obtained.
When a taking lens with a wide-angle focal length is used, a distance-measuring area is formed with a wide angle. Therefore, in some cases, the line sensors are used from end to end. Accordingly, due to the influence of a deterioration in the performance of reception lenses of an AF sensor or a degradation in the sensitivity of the line sensors, the end portions of each line sensor generate output data indicating lower brightness than that indicated by data generated from the central portion of each line sensor.
FIG. 24 shows output data of line sensors when a uniform brightness area is monitored. FIG. 24 shows the fact that the respective end portions of the line sensors generate data indicating brightness darker than that indicated by data of the central portions thereof. In this case, when distance-measuring control is performed according to a method disclosed in Japanese Unexamined Patent Application Publication No. 2001-141987, it is determined by mistake that output data of the end portions of the line sensors indicates low brightness. Disadvantageously, photographing is performed with poor distance-measuring.